from Seattle, Washington, location of the University of Washington, where the protein was first isolated and characterized
The name protein S was introduced in Richard D. Di Scipio, et al., "A Comparison of Human Prothrombin, Factor IX (Christmas Factor), Factor X (Stuart Factor), and Protein S," Biochemistry, vol. 16, no. 4 (February 22, 1977), p. 698: "The new protein was arbitrarily called protein S in reference to its isolation and characterization in Seattle. When its function is discovered, a more appropriate name can be assigned to this plasma protein."